Treatment of hydrocarbons containing acids

ABSTRACT

The invention relates to a method of treating a liquid hydrocarbon (e.g. heavy oil, bitumen, etc.) containing naphthenic acids or other corrosive acids in order to partially or fully convert such acids into non-corrosive compounds. The method comprises adding an alkylating agent to the hydrocarbon, bringing the hydrocarbon into contact with an aqueous liquid containing an alkaline compound and a phase transfer catalyst to form an immiscible two-phase system, maintaining the contact to allow conversion of the naphthenic acids to non-corrosive oil-soluble esters, and separating the aqueous phase from the liquid hydrocarbon. Naphthenic acids are highly corrosive to plant and equipment and the method enables them to be converted to non-corrosive compounds in an economic manner.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to the treatment of hydrocarbons, such as crude oil, bitumen, distillates, organic solvents, etc., containing corrosive acids. More particularly, although not exclusively, the invention relates to the treatment of hydrocarbons derived from natural sources containing naphthenic acids.

(2) Description of the Related Art

Naphthenic acids (NAs) occur naturally in crude oils, especially heavy crude oils and bitumens. The amount of acid in a hydrocarbon of this kind is usually represented by the Total Acid Number (TAN). The presence of naphthenic acids in such hydrocarbons is disadvantageous because they cause severe corrosion problems in refinery equipment, transfer pipelines, and the like. Moreover, naphthenic acids act as surfactants and disadvantageously stabilize emulsions when the hydrocarbons are contacted with water, e.g. during desalting operations. Removing naphthenic acids or significantly reducing their concentration is therefore important for heavy oil upgrading and other procedures carried out on hydrocarbons. The most common industrial practices for such treatment rely either on dilution of the hydrocarbons with low naphthenic acid feeds, thereby reducing the average TAN value of the mixture, or on a caustic washing process to convert the acids to salts and thereby neutralizing and removing the acids. Each of these methods has some disadvantages. For example, although the process of blending a high TAN crude oil with a low TAN crude oil can reduce the concentration of naphthenic acids in the blend to an acceptable level, the acidic compounds are still present, and the low TAN crude oil component is reduced in value as its level of TAN is actually increased. Furthermore, the quality of crude oil is deteriorating in general nowadays, so it will be more difficult in the future to find enough low TAN crude oil to use as blending components. On the other hand, while caustic treatments can substantially remove naphthenic acids, the procedure generates substantial amounts of waste water and results in the formation of emulsions that are difficult and expensive to treat.

There have been proposals for dealing with naphthenic acids in these and other ways. These generally fall into six categories, as summarized in the following.

(1) Catalytic Hydrogenation.

In catalytic hydrogenation, the naphthenic acids are removed by conversion to CO₂ and H₂O by the hydrogenation of heavy oils under high temperature and pressure in the presence of a hydrogenation catalyst. However, capital costs are high because of the required high pressures and temperatures, and hydrogen is expensive. In order to attain the required reaction temperature, the heavy oils have to be heated in a furnace or by heat-exchangers and the naphthenic acids in the oils will cause corrosion of such equipment.

(2) Catalytic or Thermal Decarboxylation.

This method has drawbacks similar to those of catalytic hydrogenation. In fact, the reaction temperatures required are higher than those required for the hydrogenation reactions. Decarboxylation catalysts also often have poor stability.

(3) Neutralization With Alkaline Aqueous Solutions.

As well as the problems discussed above (e.g. the formation of stable emulsions), the naphthenic acids are converted to salts (naphthenates) and transferred to the aqueous phase. The oil yield decreases in consequence and the naphthenates in the aqueous phase must be treated further (recovered or converted), which is difficult and costly. An example of such a process is disclosed in PCT patent publication WO 01/79386 published on Oct. 25, 2001 to Mark Greaney. This publication discloses a process of reducing naphthenic acid content of crude oils in the presence of an aqueous base and a phase transfer agent at elevated temperature and pressure to produce water-soluble naphthenate salts. Other such processes are disclosed in U.S. Pat. No. 6,627,069 issued to Mark Alan Greaney on Sep. 30, 2003 and PCT patent publication WO 00/75262 published on Dec. 14, 2000 to Collins et al.

(4) Extraction With Tetraalkylammonium Salts, Amines or Other Extractants.

This method has the same drawbacks as the neutralization method because the naphthenic acids are converted to other compounds and removed.

(5) Esterification With Alcohols in the Presence of Catalysts.

This method requires homogenous or heterogeneous catalysts. The separation of homogeneous catalysts from the products is generally very difficult, and remaining catalyst may lead to many other unwanted side reactions. Also, by this method, the conversion of naphthenic acids is very low if the water produced by the esterification reaction cannot be removed from the reaction system (the presence of water so-produced inhibits the conversion).

(6) Other Processes

U.S. Pat. No. 5,683,626 which issued to Sartori et al. on Nov. 4, 1997, for example, discloses a process in which crude oil is contacted with a neutralizing amount of tetraalkylammonium hydroxide to produce naphthenic esters. However, the oil has to be heated to a temperature in the range of 50 to 350° C. for many hours.

Due to the corrosion potential of naphthenic acids, it is desirable to remove the acids at the location where the hydrocarbons are initially obtained, but this is often difficult with at least some of the above methods because it is uneconomic to provide suitable equipment in the inaccessible or remote locations where crude oils bitumens are often found.

Despite the various known approaches to the problem, there is therefore still a need for an improved method of dealing with naphthenic acids present in hydrocarbons such as heavy oils and bitumen.

BRIEF SUMMARY OF THE INVENTION

According to one exemplary embodiment of the present invention, there is provided a method of treating a liquid hydrocarbon containing acids, which comprises bringing the hydrocarbon into contact with an aqueous liquid containing an alkaline compound and a phase transfer catalyst in the presence of an alkylating agent to form a two-phase system, maintaining the contact to allow conversion of the acids to non-corrosive oil-soluble esters, and separating the aqueous phase from the liquid hydrocarbon. The alkylating agent is preferably added to the hydrocarbon before or simultaneously with the contact with the aqueous liquid.

The acids in the hydrocarbons are generally those referred to as naphthenic acids when the hydrocarbons are naturally occurring (i.e. originally derived from naturally-formed geological reservoirs and often referred to as “fossil fuels”). However, the process may be used for removing organic acids of other kinds from water-immiscible hydrocarbons in general, including man-made or recycled hydrocarbons such organic solvents. Most preferably, the method is applied to naphthenic acid-containing bitumen, heavy oil, whole crude or topped crude, and distillates from the bitumen or crude or from secondary processes. Bitumen may contain other components, such as fines and asphaltenes, and may then be difficult to treat efficiently. To overcome this problem, bitumen may first be diluted with a solvent or light fraction (e.g. naphtha) to reduce its viscosity.

The method may be applied to hydrocarbons having any TAN value above zero. Usually, the TAN value in crude oils is greater than 0.5. In sidestreams considered to be corrosive, the TAN value is often greater than 1.5. Clearly, only hydrocarbons considered to be corrosive (in their present form or after subsequent treatment) need to be subjected to the method of the exemplary embodiments.

The alkylating agent is preferably an alkyl halide (preferably a bromide or a chloride) or an alcohol. The alkylating agent should preferably have some degree of oil solubility so that it can be continuously transferred to the oil phase as the agent is consumed by the esterification reaction. If the oil solubility is low, the rate of reaction may be slow. Therefore, when using choosing water soluble alklyating agents, e.g. lower alcohols, those having a low carbon number (e.g. methanol) should preferably be avoided.

The molar ratio of the alkylating agent to the acid (e.g. as calculated from the TAN) is preferably in a range of 1-10:1.

The alkaline compound used in the method is preferably a base selected from NaOH, KOH, Ca(OH)₂, ammonium hydroxide, and mixtures thereof, but other bases could possibly be employed. The concentration of the alkaline compound in the aqueous solution is preferably 0.001-1.0 M.

The phase transfer catalyst is preferably a compound having the formula:

[XR¹R²R³R⁴]Y

wherein: X is N or P (and most preferably N);

-   -   Y is Cl, Br, I or OH (most preferably Br); and     -   R¹, R², R³ and R⁴ may be the same or different and each         represents an alkyl group having a carbon number in the range of         1 to 20.

Examples of suitable phase transfer catalysts of this kind include cetyltrimethyl ammoninum bromide (CTAB), didodecyltrimethyl ammonium bromide, tetrabutylammonium bromide, tetrapentylammonium bromide and tributylhexadecaylammonium bromide.

Phase transfer catalysts of this kind may be readily obtained from commercial sources, such as for example PTC Organics Inc. of New Jersey, USA, or Sigma-Aldrich of St. Louis, Mo., USA.

The catalyst is preferably used in an effective amount normally in the range of 0.05 to 10 wt. % of the aqueous phase.

The reactants are preferably stirred during the reaction to achieve a good mixing of the hydrocarbon and aqueous phases. This may be carried out by bringing the phases together in a stirred tank or similar agitated vessel. The stirring speed may determine the mass transfer, so the higher the speed is, the better. The method may be continuous or carried out in batches of any convenient size. Since emulsions are not generally formed to any substantial extent, the aqueous phase may be separated from the oil phase after the reaction is complete simply by allowing the mixture to stand until two discrete layers are formed, and then decanting or drawing off one layer from the other. If necessary or desirable, a centrifuge may be used for the separation.

The method is generally carried out at mild temperature, e.g. below 120° C. and more preferably below 100° C. A preferred temperature range is 30 to 100° C., and more preferably 40 to 90° C. However, the reaction may be carried out at ambient temperatures (such as 5° C. to 35° C.) or at room temperatures (such as 15 to 25° C.) so that no heating equipment may be required.

As the reaction takes place in liquid phases, pressure is not usually relevant and the method may be carried out at atmospheric pressure.

The time required for the reaction is preferably 10-240 minutes, and more preferably 30-120 minutes. Longer periods of time may be employed, but are generally not required.

As the method requires a simple operation under mild conditions, and may be carried out in simple apparatus (stirred tanks, and the like), it may be used to remove naphthenic acids from hydrocarbons in remote locations where the hydrocarbons are first collected. This minimizes or eliminates corrosion in down-stream plant and equipment.

Definitions

The term “naphthenic acids” as used herein refers generically to the acids found in naturally-occurring hydrocarbons such as crude oil, bitumen, etc. These acids are usually an unspecific mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic units. The main fraction are often carboxylic acids with a carbon backbone of 9 to 20 carbons.

The term “TAN” is an acronym for Total Acid Number and the “TAN value” of a hydrocarbon is the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of the hydrocarbon.

The term “phase transfer catalyst” or “PTC” refers to a catalyst which facilitates the migration of a reactant in a heterogeneous system from one phase into another phase where reaction can take place. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase unless a phase transfer catalyst is present. Phase transfer catalysts for anion reactants are often quaternary ammonium salts, but other species may be effective. Further information about phase transfer catalysts may be obtained from Mieczyslaw Makozal and Michal Frdorynski, “Phase Transfer Catalysis”, CATALYSIS REVIEWS, Vol. 45, Nos. 3 and 4, pp. 321-367, 2003 (the disclosures of which is specifically incorporated herein by this reference).

The term “alkylating agent” means a reactant that is capable of reacting with naphthenic acids to convert them to corresponding esters. Alkyl radicals generally contain only carbon and hyrdrogen, and may be saturated or unsaturated. Substituted alkyl radicals may be employed in the present invention provided that any additional elements or radicals present do not adversely affect the esterification reaction nor make the resulting ester oil-insoluble.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in further detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of an interfacial region between an oil phase and a water phase illustrating the kind of reactions that may take place in the exemplary embodiments of the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

An exemplary embodiment of the present invention makes use of a phase transfer catalyst (PTC) that allows compounds to transfer rapidly from one phase to another as reactions occur, e.g. during the esterification of naphthenic acids originally present in the hydrocarbon with an alkylating agent such as an alkyl halide or an alcohol. The use of such catalysts has the advantage that the desired reactions may be brought about at relatively low temperatures (e.g. below 100° C., and often at ambient temperature such as, for example, 15 to 30° C.), and that products of the reaction (naphthenic esters) remain in the oil phase but are non-corrosive. If necessary, the esters may be removed from the hydrocarbon during further refining, e.g. by distillation. The catalyst remains in the aqueous phase and can be reused. A trace amount of the catalyst may transfer to the oil phase during the method but, if so, it can easily be removed in subsequent processing (e.g. by washing the oil phase with water) without causing any corrosion or environmental problems. If desired, the catalyst may eventually be recovered from the aqueous phase.

FIG. 1 of the accompanying drawings illustrates one preferred embodiment of the present invention. The drawing shows an oil-and-water system 10 consisting of an oil phase layer 14 and an aqueous phase layer 16. The aqueous phase layer 16 has an interface layer 12 in immediate contact with the oil phase layer 14. In practice, the interface layer 12 is a very thin boundary layer forming part of the aqueous phase layer. This is the region where diffusion of species between the two phases may take place at a rapid rate. The oil phase may be, for example, heavy crude oil, and the aqueous phase may be a continuous layer or a droplet of an aqueous liquid brought into contact with the oil phase. Naphthenic acid (represented in the drawing as RCOOH, although in practice it will be a mixture of many kinds of acids) is present in the oil phase layer 14 as shown. The naphthenic acid reacts with an alkali (in this case, sodium hydroxide, NaOH) present in the aqueous phase layer 16 to form naphthenic anions (RCOO⁻) which, because of their ionic charge, diffuse rapidly to the aqueous phase at the interface 12. The aqueous phase also contains a phase transfer catalyst, in this case an alkyl quaternary ammonium bromide represented as R′₄NBr (the four groups R′ may be the same or different, and may be the same as or different from R in the representation of the naphthenic acid. Presently preferred catalysts are [(C₄H₉)₄N]Br and [(C₄H₉)₃—N—C₁₆H₃₃]Br). The bromide dissociates to form a quate cation that reacts with the naphthenic anion to form RCOO—NR′₄ which is highly soluble in the oil phase layer and quickly migrates there. In turn, the oil phase has been provided with an at least partially oil-soluble alkylating agent, here shown as C₄H₉Br, that reacts with the RCOO—NR′₄ to form a naphthenic ester RCOOC₄H₉ that is soluble in the oil phase layer and dissipates into that layer. The phase transfer catalyst is regenerated and moves to the aqueous layer at the interface 12 where it is again available for reaction with the naphthenic anion. The reaction is cyclic as the phase transfer catalyst is continuously regenerated and reused, and may take place very quickly. Very little catalyst is lost over time, so catalyst costs are kept very low. The naphthenic acids are converted without significant oil loss (because the ester remains as an oil constituent) and without the production of an emulsion (even though an alkali is present). Conversions up to 100% may be achieved in favorable circumstances.

The oil phase and aqueous phase are preferably agitated together (e.g. stirred or shaken) to promote a rapid transfer of chemical species both within the bulk of the phases and across the interface. Preferably, the aqueous phase is provided in an amount in the range of 1-20 volume % of the total oil and water mixture.

In the illustrated embodiment, bromide is used as the halide of both the PTC and the alkylating agent. However, the halide (or other equivalent species such as —OH) does not have to be the same for both the catalyst and the alkylating agent.

In order to carry out the indicated process, the phase transfer catalyst (e.g. tetralkyl ammonium halide) is dissolved in alkaline hydroxide (e.g. NaOH or KOH) solution and then mixed with the hydrocarbon to which an alkylating agent (alkyl halide) has been added. The mixture is preferably stirred for a suitable time (normally 10 to 240 minutes). After the stirring is terminated, the aqueous and oil phases separate readily, and the aqueous phase can be easily removed as no emulsion is formed. The alkali is preferably used at a concentration up to 1.0M, preferably 0.5M, and possibly up to 0.1M. The alkaline water content is generally 1 to 20 wt. %. As previously noted, the temperature of the reaction is generally kept below 100° C., and the reaction is generally carried out under atmospheric pressure.

The method is advantageously carried out as soon as the hydrocarbon is available, e.g. before it is transported through pipelines to the next processing stages, e.g. desalting, heating, distillation, etc. In this way, corrosion of the equipment used for such transportation or subsequent processing can be significantly reduced. The removal of naphthenic acids before any desalting is particularly preferred because this greatly reduces the formation of emulsions in the desalting apparatus and makes the oil-and-water separation much easier. This not only increases the amount of oil recovery, but also reduces energy costs. Moreover, since the naphthenic acids are converted to esters that remain in the treated oil phase, there is nothing to remove from the aqueous phase and no further treatment of conversion products is required.

Further exemplary embodiments of the invention are described in the Examples below. These Examples are provided for the purpose of illustration only.

EXAMPLES

In the following Examples, all chemicals were purchased from Sigma Aldrich® and used without alteration. The properties of the HVGO used are listed in Table 1. All experiments were conducted following the procedure below:

-   -   1. Bromobutane and heavy vacuum gas oil (HVGO) were added in a         flask. The mixture was heated to 80° C. in an oil bath under         stirring.     -   2. PTC (tetrabutylammonium bromide) was completely dissolved in         NaOH aqueous solution, and added to the flask prepared in 1.     -   3. The above mixture was stirred at 80° C. for 4 hours.     -   4. After reaction, the flask was quenched with cold water and         the contents transferred into a centrifuge bottle.     -   5. The water was separated from oil using an IECCR-6000®         centrifuge (3800 rpm for 30 min).

The oil phase was analyzed for TAN, sulfur, nitrogen, and boiling range. The nitrogen and sulfur were analyzed according to ASTM D4629 (syringe/inlet oxidative combustion and chemiluminescence detection), and ASTM D4294 (energy-dispersive X-ray fluorescence spectrometry), respectively. The boiling range was measured with simulated distillation (SimDis) by gas chromatography. The water was tested for pH as well as carbon and nitrogen contents. The carbon and nitrogen contents in water phase were analyzed by ICP. The results are shown in Table 1 below.

TABLE 1 Properties of HVGO TAN 4.12 Sulfur, wt % 3.28 N, wt % 1.22 Boiling range ° C.  0.5 262 10 325 30 369 50 404 70 438 80 459 90 488 95 510 99 551 99.5 566

Various tests were carried out at different NaOH concentrations as illustrated in Table 2 below.

TABLE 2 NaOH concentration effect Sample No. ENR-15 ENR-18 ENR-20 ENR-21 Reactant composition HVGO, g 80 80 80 80 NaOH solution Weight, g 20 20 20 20 Concentration of NaOH, M 0.001 0.15 0.25 0.50 PTC, g 3.55 3.55 3.55 3.55 Bromobutane, g 7.54 7.54 7.54 7.54 Reaction conditions Temperature, ° C. 80 80 80 80 Time, hours 4 4 4 4 Treated oil TAN 3.58 2.61 0.35 0 Naphthenic acid removal, % 13.1 36.6 91.5 100 Ease of oil-water separation Easy Easy Easy Easy Water phase appearence Clear Clear Clear Clear

The TAN values after various reaction time were measured and the results are shown in Table 3 below.

TABLE 3 TAN after various reaction time, mgKOH/g oil NaOH concentration, M Reaction time, min 0.15 0.25 30 1.598 0.330 60 1.638 0.355 120 1.668 0.350 Temperature: 80° C. NaOH solution: 20 g Bromobutane: 7.54 g HVGO: 80 g PTC: 3.55 g

The effects of different concentrations of the PTC were determined and the results are shown in Table 4 below.

TABLE 4 Effect of PTC concentration Sample No. ENR-34-1.0 ENR-34-0.5 ENR-34-0.2 PTC/acid molar ratio 1.0 0.5 0.2 Reactant composition HVGO, g 80 80 80 0.25M NaOH solution, g 20 20 20 PTC, g 1.89 0.95 0.38 Bromobutane, g 7.54 7.54 7.54 Reaction conditions Temperature, ° C. 80 80 80 Time, hours 4 4 4 TAN of treated oil, mgKOH/g 0.501 0.618 0.481 Ease of oil-water separation Easy Easy Easy Water phase appearance Clear Clear Clear

Different PTCs were used in the method and the results are shown in Table 5 below. It can be seen that tributylhexdecaylammonium bromide gave the better result, although both results were acceptable.

TABLE 5 Effect of PTC type Tetrabutyl- ammonium Tributylhexdecaylammonium PTC catalyts bromide bromide Reactant composition HVGO, g 80 80 0.25M NaOH solution, g 20 20 PTC, g 3.55 3.55 Bromobutane, g 7.54 7.54 Reaction conditions Temperature, ° C. 80 80 Time, hours 4 4 TAN of treated oil, 0.35 0.05 mgKOH/g Ease of oil-water separation Easy Easy Water phase appearance Clear Clear 

1. A method of treating a liquid hydrocarbon containing acids to reduce corrosive properties thereof, which method comprises bringing the hydrocarbon into contact with an aqueous liquid containing an alkaline compound and a phase transfer catalyst in the presence of an alkylating agent to form a two-phase system, maintaining the contact to allow conversion of the acids to non-corrosive oil-soluble esters, and separating the aqueous phase from the liquid hydrocarbon.
 2. The method of claim 1, wherein the hydrocarbon is naturally occurring and said acids are naphthenic acids.
 3. The method of claim 2, wherein said hydrocarbon is selected from the group consisting of bitumen, heavy oil, whole crude, topped crude and distillates thereof.
 4. The method of claim 1, wherein the phase transfer catalyst has the formula: [XR¹R²R³R⁴]Y wherein: X is N or P (and most preferably N); Y is Cl, Br, I or OH; and R¹, R², R³ and R⁴ may be the same or different and each represents an alkyl group having a carbon number in the range of 1 to
 20. 5. The method of claim 1, wherein the phase transfer catalyst is a tetraalkylammonium compound.
 6. The method of claim 1, wherein the catalyst is selected from the group consisting of tetrabutylammonium bromide and tributylhexadecaylammonium bromide.
 7. The method of claim 1, wherein the alkylating agent is an alkyl halide.
 8. The method of claim 1, wherein the alkylating agent is an alcohol other than methanol.
 9. The method of claim 1, wherein said hydrocarbon is maintained at a temperature of 0 to 120° C. during said contact.
 10. The method of claim 1, wherein said hydrocarbon is maintained at a temperature of 40 to 90° C. during said contact.
 11. The method of claim 1, wherein said contact is maintained under atmospheric pressure.
 12. The method of claim 1, wherein said contact is maintained for a period of time in a range of 10 to 240 minutes.
 13. The method of claim 1, wherein said contact is maintained for a period of time in a range of 30 to 120 minutes.
 14. The method of claim 1, wherein the hydrocarbon and aqueous liquid are agitated during said contact.
 15. The method of claim 1, wherein the alkaline compound is selected from the group consisting of NaOH, KOH, Ca(OH)₂, ammonium hydroxide and mixtures thereof.
 16. The method of claim 1, wherein the alkaline compound is used at a concentration up to 1.0 M.
 17. The method of claim 1, wherein the alkaline compound is used at a concentration in a range of 0.001 to 1.0 M.
 18. The method of claim 1, wherein the catalyst is used in an amount in a range of 0.05 to 10 wt. % of the aqueous phase.
 19. The method of claim 1, wherein a molar ratio of the alkylating agent to the acids is 1-10:1.
 20. The method of claim 1, wherein said alkaline liquid is used in an amount of 1 to 20 volume % of the total amount of said hydrocarbon and aqueous liquid. 